1. Field
This disclosure relates generally to an amplifier circuit, and more specifically to a differential amplifier circuit having a dynamically biased configuration.
2. Related Art
An amplifier circuit (or a buffer circuit (buffer)) can be used to adapt the output characteristics of one circuit to the input characteristics of another circuit. A buffer circuit can be considered an amplifier circuit with unity gain. In one example, a voltage buffer circuit can be used to transfer a voltage from a first circuit that has a high output impedance level, to a second circuit with a low input impedance level. Thus, the buffer prevents the second circuit from loading the first circuit unacceptably (e.g., with a large current load) and thus interfering with the desired operation of the first circuit.
Preferably, a buffer would not be needed between portions (e.g., first and second circuits) of a system. However, when a buffer circuit is needed, it is a goal for the design of the buffer circuit to minimize current consumption. By reducing current consumption, power consumption can be reduced, which is particularly important for portable battery powered devices that need extended operating time.
The inputs and outputs of cooperating circuits can be connected with differential signals or differential signaling. Differential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires or signal paths. At the receiving end of the transmission, the information is read or interpreted as the difference between the two complementary signals. Differential signaling increases noise immunity by rejecting common-mode interference, which is the interference that exists on, or is common to, both signal paths.
In amplifier circuits, or buffer circuits (e.g., a unity gain amplifier circuit), a current source (or a current sink, which may be thought of as a negative current source) can be used to bias transistor circuits. In an ideal current source, the current through the source remains constant as the voltage across the source changes. It is frequently desirable to increase the output resistance of the current source in order to make the current source more ideal. One way of increasing the output resistance of a current source is to add a cascode transistor, where the cascode transistor typically has a fixed voltage applied to the control input (e.g. the base or gate terminal) of the transistor.